Rubber tires have been manufactured using essentially the same process for about one hundred years. The process includes the following steps: Rubber is mixed with other chemicals on mills or in banburys, flat rubber components are formed with calenders, contoured rubber components are formed with calenders or extruders, reinforcing fabrics and wires for plies and belts are coated with rubber on a calender train and subsequently cut to bias angles, solid wire is insulated with rubber on an extruder and subsequently wound into beads, all components are assembled on a building drum or mandrel, the assembled tire is placed in a hot mold having the shape of the finished tire including tread pattern and expanded out to the mold surface by internal pressure, after significant time has passed the completely vulcanized tire is removed from the mold and is ready for service.
Vulcanization time varies according to tire size and amount of rubber. Vulcanization times vary from ten minutes for passenger tires to fifteen hours for large earthmover tires.
A rubber tire must remain in the hot mold under pressure until every particle of rubber is vulcanized. Heat from the mold and from inside the tire slowly reaches the center of mass because rubber is a thermal insulator having low thermal conductivity. The tread component of a tire is one of the more massive components thereby slowing heat transfer to the center of tire mass and increasing the time that a tire must remain in the mold. Longer vulcanization time in a mold increases the number of molds required for the same output.
A rubber treaded tire must be built smaller than the inside of the mold in order to fit into the tread pattern forming part of the mold and then after the mold is closed the tire is expanded with high internal pressure until rubber flows into the mold's tread pattern sometimes causing flow cracks or imperfectly molded tread lugs and internal stresses which adversely affect field performance.
An inflated vulcanized rubber treaded tire can never be as round as the mold in which it was vulcanized because it was molded with high internal pressure but operates in the field at significantly lower inflation pressure and its roundness is no longer controlled by the mold but by the reinforcing plies, belts and beads within its structure. Out of round tires do not ride as smoothly as round ones.